Conventionally, in the field of gasoline engines, a general combustion method (spark ignition combustion) is a method in which a fuel-air mixture is forcibly ignited by spark ignition of a spark plug. However, in recent years, instead of the spark ignition combustion, studies of applying so-called homogeneous-charge compression ignition combustion to the gasoline engines have been undertaken. According to the homogeneous-charge compression ignition combustion, fuel injected into a cylinder before compression top dead center is mixed with air, and the prepared air-fuel mixture is ignited by itself (not by the spark ignition) under a high-temperature environment generated by compression of a piston. According to the homogeneous-charge compression ignition combustion, plural combustion phenomena of the air-fuel mixture simultaneously occur in the cylinder. Therefore, it is said that a combustion period of the homogeneous-charge compression ignition combustion is shorter than a combustion period of the combustion by the spark ignition, and a heat efficiency of the homogeneous-charge compression ignition combustion is higher than a heat efficiency of the combustion by the spark ignition. Hereinafter, the homogeneous-charge compression ignition combustion may be abbreviated as HCCI combustion.
To further improve fuel efficiency of a homogeneous-charge compression ignition engine, it is effective to execute a so-called idling stop control of automatically stopping and restarting the engine.
One known technology regarding the above idling stop control is, for example, PTL 1 below. PTL 1 discloses that in a multi-cylinder diesel engine which combusts light oil by compression ignition (diffusion combustion) or in a multi-cylinder gasoline engine which combusts gasoline by compression ignition (homogeneous-charge combustion), a control mode when restarting the engine is changed based on a stop position of the piston of a specific cylinder.
Specifically, in PTL 1, when the engine is automatically stopped, a stop position of a piston of a stopped-in-compression-stroke cylinder that is in a compression stroke at that time is examined. When a restart condition of the engine is satisfied, whether or not the stop position of the piston of the stopped-in-compression-stroke cylinder is located at a bottom dead center side of a predetermined position is determined. When the stop position of the piston of the stopped-in-compression-stroke cylinder is located at the bottom dead center side of the predetermined position, the fuel is firstly injected into the stopped-in-compression-stroke cylinder, and the engine is started by restarting the combustion from first compression in which the compression top dead center is reached for the first time in the entire engine, that is, by restarting the combustion from the cylinder in which the compression top dead center is firstly reached among all the cylinders (four cylinders in PTL 1) (hereinafter referred to as a “first compression start”).
In contrast, when the stop position of the piston of the stopped-in-compression-stroke cylinder is located at a top dead center side of the predetermined position, the fuel is firstly injected into a cylinder (stopped-in-intake-stroke cylinder), which is in an intake stroke when the engine is stopped, after the stopped-in-intake-stroke cylinder shifts to the compression stroke. Thus, the engine is started by restarting the combustion from second compression in which the compression top dead center is reached for a second time in the entire engine, that is, by restarting the combustion from the cylinder in which the compression top dead center is secondly reached among all the cylinders (hereinafter referred to as a “second compression start”).